Subsurface safety valves are placed in tubular strings to allow shutting in the well for well control. They are actuated to open for production or injection with hydraulic control lines that extend from a surface location. One or two line systems have been used for valve control. The hydraulic pressure operates a rod or annular piston in the housing of the valve. The piston is linked to a flow tube in the passage through the valve. The piston is driven with hydraulic pressure in a control line against the bias of a closure spring. Movement of the piston against the spring takes the flow tube against the flapper that is biased with a pivot spring to a closed position. On contact with the flapper, the pivot spring is overcome as the flapper rotates 90 degrees and the flapper goes behind the advancing flow tube. The open position is maintained as long as pressure is supplied in the hydraulic control line. One removal of such pressure deliberately or through a system malfunction such as a seal leak the closure spring takes over and raises the piston that takes the flow tube with it to allow the pivot spring to move the flapper to the closed position.
When the flapper is in the closed position there is a large pressure differential potential that can appear across it making it difficult for the hydraulic system to provide the required force to open the flapper with the flow tube without component damage. To address this problem in the past equalizer plungers have been placed in the flapper at a location where the descending flow tube would engage the plunger to open a bypass passage through the flapper before the flow tube was brought into contact with the flapper itself for rotation to the open position. In this manner the pressure across the flapper was equalized before the flow tube was in contact with it to rotate it. In this case only the force of the pivot spring needed to be overcome to open the flapper. The prior designs all employed springs to return the equalizer plunger back to a sealed position when the flow tube no longer contacted the plunger. This was done with leaf or coil springs as illustrated in these U.S. Pat. Nos. 4,478,286; 6,644,408; 7,204,313; 6,848,509; 4,415,036 and 8,056,618. Referring specifically to U.S. Pat. No. 7,204,313 the problem was that the spring 86 would not stay mounted to the plunger 86 by moving off its mount flange 70. Another issue was that the shape and length of the spring resulted in a very minimal closing force applied to the plunger in the order of about two pounds. One of the reasons that the spring could be moved off its flange mount is that the plunger is normally struck by the flow tube in an off center manner which could have put a slight turning moment on the plunger sufficient to dislodge the spring from its mounting flange. Without the spring in position to push the plunger to its closed position the safety valve became inoperative as there was a perpetual bypass flow through the flapper in the closed position. This was a safety issue that needed to be addressed.
The present invention addresses these concerns with a design where the spring is integrally fabricated in the plunger body using techniques known as wire EDM (Electrical Discharge Machining) or more familiarly “turn and burn.” As a result a spring is integrated into the plunger that can put out substantially higher closing force without taking up incremental space and this allows less of the flapper body to be removed. The integral design can be symmetrical or asymmetrical and the plunger in an asymmetrical design can be keyed to prevent rotation and the spring designed to generate more force on one side to better counter act the side loading force from off-center contact from the flow tube. These and other features of the invention will be more apparent to those skilled in the art from a review of the detailed description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be found in the appended claims.